This invention relates generally to transmission systems for a dispersive medium and particularly relates to a system for encoding the digital information with different degrees of redundancy and demodulating the received information with the maximum likelihood of detection.
In recent years it has become of increasing importance to be able to transmit digital information over a dispersive channel while still recovering the original information. The digital information may, for example, represent audio information, such as voice, or video information.
To this end it has been proposed in the past to transmit by channel vocoder or linear predictive coding processors. Recently, adaptive predictive coding processors have become known. Such adaptive predictive coding has been disclosed in a paper by Atal et al. which appears in the Bell System Technical Journal, October, 1970, pages 1973 to 1986. In this connection reference is made to another paper by Bayless et al. entitled "Voice Signals: Bit-By-Bit" which appears in IEEE Spectrum, October 1973, pages 28 - 34.
As a result of the adaptive predictive coding, for example of speech, the resulting bits may be classed into groups of bits which are more or less important for the transmission of the information. For the most important bits the tolerable bit error rate may be as low as 0.001 and for the next most important bits the bit error rate may be 0.01. Finally, for the residue bits of the coding of the signals a bit error rate as high as 0.10 may be tolerated. Instead of the adaptive predictive coding disclosed in the two papers above referred to, it is also feasible to provide feedback reesidue compression for digital speech. Such a system has been disclosed in the copending application to Sandra E. Hutchins, Ser. No. 612,992 filed on Sept. 12, 1975.
It is therefore desirable, and proposals have been made in the past for encoding the digital information by means of cyclic group codes having more or less redundancy as required by the type of information.
It is well known that the information provided in the form of cyclic group codes can be decoded by means of various error correcting schemes. One such error correcting decoder for cyclic group codes has been disclosed in the patent to Solomon U.S. Pat. No. 3,818,442 which issued on June 18, 1974.
However, when the encoded digital information is transmitted through a dispersive medium, for example by high frequency carriers (from 3 - 30 megahertz), this type of hard decision decoding will result in many errors. The typical dispersive channel may delay some of the information as a function of frequency and it may provide a time variable and frequency variable gain or rather loss. This may be due to the multipath of the information and frequency selective fading. Hence, the received information is of a time varying type in accordance with frequency and other factors to which additionally may be added the white noise of the channel.
In order to deal with this problem it has been proposed to decode the information say for a present frame n by looking back on the previous frame (n-1). Such a system has been proposed by Chase in a paper which appears in IEEE Transactions on Communications, Vol. COM-21, No. 3, March 1973, pages 159 - 174. As outlined in this paper an initial decision, called a hard decision, on the received vector is made, the decision consisting of determining each code symbol separately. The data concerning the physical signal and the interrelations among the symbols in each code word are now utilized to correct the first decision. The later decision which is sometimes called a soft decoding makes use of an interative scheme whereby the number of iterations is undeterminate. As a result, it may either take too long a time to obtain a final decision or the number of iterations may have to be limited and the initial decision accepted so that the error correction capability is reduced.
It is accordingly an object of the present invention to provide a communication system for digital information utilizing error correcting coding which is modulated onto a plurality of tones or subcarriers.
A further object of the present invention is to utilize for a system of the character discussed an 8-ary Reed Solomon code of the type (7,k) where the 7 represents the length and the k the dimension of the code, so that by varying k the redundancy of the code may be varied for bits of greater or lesser importance.
Another object of the present invention is to decode the transmitted information by first decoding the received information to generate a plurality of candidate code words and thereupon by correlation techniques to obtain maximum likelihood of detection with a fixed number of steps or iterations.
Still a further object of the present invention is to provide a system for transmitting and receiving digital information where the original information is compressed so that it can be transmitted over a frequency restricted channel while still maintaining the fidelity of the original information.